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Kilonova fireball provides evidence of gamma ray bursts

Jenny Winder, News Writer
Aug 5, 2013, 7:00 UTC

Sen—The Hubble Space Telescope has observed the fading fireball produced in the aftermath of a short gamma-ray burst (GRB).

The afterglow reveals a new kind of stellar blast, called a kilonova, providing evidence that such GRBs are triggered by the merger of two super-dense stellar objects, a pair of neutron stars or a neutron star and a black hole.

Gamma-ray bursts are flashes of intense high-energy radiation. Short-duration blasts last at most a few seconds, but they sometimes generate afterglows in visible and near-infrared light that continue for several hours or days.

Such afterglows helped astronomers determine that GRBs lie in distant galaxies. The cause of short-duration GRBs remains a mystery. The most popular theory is the energy released as two compact objects crash together. But, until now, astronomers could not prove it.

Researchers led by Nial Tanvir of the University of Leicester in the United Kingdom used Hubble to study a recent short-duration burst in near-infrared light, revealing the afterglow of a kilonova explosion, the "smoking gun" evidence for the merger hypothesis.

"Many astronomers, including our group, have already provided a great deal of evidence that long-duration gamma-ray bursts (those lasting more than two seconds) are produced by the collapse of extremely massive stars. But we only had weak circumstantial evidence that short bursts were produced by the merger of compact objects. This result now appears to provide definitive proof supporting that scenario."

When a pair of super-dense neutron stars in a binary system spiral together, the system emits gravitational radiation, ripples in the fabric of space-time. The energy dissipated causes the two objects to sweep closer together. As they merge, they kick out highly radioactive material that heats up and expands, emitting as much visible and near-infrared light every second as the Sun does every few years.

In a recent science paper Jennifer Barnes and Daniel Kasen of the University of California and the Lawrence Berkeley National Laboratory predicted that the hot plasma producing the radiation will block the visible light, causing the energy from the kilonova to flood out in near-infrared light over several days.

In the image at left, the galaxy in the centre produced GRB 130603B. Hubble's Wide Field Camera 3 on June 13, 2013, revealed a glow in near-infrared light at the source of the GRB, shown in the image at top, right. When Hubble observed the same location on July 3, the source had faded, shown in the image at below, right. Image credit:  NASA/ESA/ N. Tanvir (University of Leicester) A. Fruchter (STScI) A. Levan (University of Warwick)

On June 3 NASA's Swift Space Telescope picked up the gamma-ray burst, GRB 130603B, in a galaxy 4 billion light-years away. The initial blast of gamma rays lasted just one-tenth of a second, but was roughly 100 billion times brighter than the subsequent kilonova flash.

On June 12-13 Hubble searched the location of the initial burst, spotting a faint red object. Subsequent Hubble observations three weeks later, on July 3, revealed the source had faded away, providing the key evidence it was the fireball from an explosive event.

"Previously, astronomers had been looking at the aftermath of short-period bursts largely in optical light, and were not really finding anything besides the light of the gamma-ray burst itself," explained Andrew Fruchter of the Space Telescope Science Institute in Baltimore, Maryland. "But this new theory predicts that when you compare near-infrared and optical images of a short gamma-ray burst about a week after the blast, the kilonova should pop out in the infrared, and that's exactly what we're seeing."